A better understanding is needed of the development of adult second-language speech production and perception abilities across a range of listening and speaking conditions. In order to communicate effectively, especially in adverse conditions, non-native speakers must make appropriate use of a range of cues to speech sound identification in the second language. Thus, performance across differing conditions may be viewed as reflecting non-natives' productive and perceptual phonetic knowledge of these cues, and development can be viewed as changes in cue weighting. These issues will be explored using production and perception data from high- and low-proficiency Spanish speakers of English as a second language and native English speakers. Experiment 1 will investigate the acoustic properties of vowels produced in citation and hyperarticulation conditions. Experiment 2 will investigate subjects' identification of natural vowels and synthetic vowels manipulated to neutralize or maintain the cues of duration and spectral change. Experiment 3 will measure the intelligibility of the citation and hyperarticulated vowels produced in Experiment 1. The data will be used to test the hypothesis that less proficient non-natives lack knowledge of the full range of cues to vowel identification. It is predicted that, for some cues, the less proficient non-natives will exhibit no difference in production performance between citation and hyperarticulation conditions and no difference in perception performance between the full-cue and some cue-neutralized conditions. A second prediction is that the more proficient non-natives' developing knowledge of these cues will be reflected by an exaggerated or restricted degree of change across conditions for some production and perception measures. These data will contribute to our understanding of the relationship between productive and perceptual phonetic knowledge. The data should provide some support for one of three ideas: 1) that perception unidirectionally influences production; 2) that production and perception influence one another bi-directionally; or 3) that perception and production skills develop independently. The outcome of this research should lead to more effective training methods to improve second-language learners' communication skills.